1. Technical Field
The present invention relates generally to the field of epoxy-ceramic polymer composites and specifically to the field of epoxy-ceramic polymer composites having high dielectric constants which are suitable for use in forming capacitors.
2. Related Art
New materials with high dielectric constants and low loss tangents are needed in the electronics industry for use at high frequencies and as a means to enable further miniaturization. In a typical electronic system today, the number of discrete passives, non-active electrical elements, outnumbers the active integrated circuits (ICs) by several times and can occupy more than 70% of the real estate of substrate. Discrete passives have become the major barrier to the miniaturization of electronic systems. Integration of passives in IC packages, however, has the following benefits: better electrical performance, higher reliability, lower cost, and improved design options.
Because of the large number of capacitors needed in modem electrical systems, integration of capacitors is of high importance. At the same time, the development of microelectronics requires decoupling capacitors with higher capacitance and shorter distance from their serving components. Table 1 shows predicted decoupling capacitor value requirements over the decade spanning 1995-2005.
Development of microelectronics has driven and will continue driving integrated circuit (IC) technology advances to higher levels. Meanwhile, electronic packaging has undergone tremendous development in all its aspects to satisfy the requirement of IC development. The evolution of electronic packaging can be categorized into three generations. The first generation of package, so called the discrete board package, which used discrete components that took more than 80% of the board area to fulfill the supporting function to IC. The second generation used technologies such as chip scale packaging (CSP) and multi-chip-module (MCM) to increase the IC efficiency to 30-40%. The third generation, system on package (SOP), will be based on single level integrated module (SLIM) technology (as shown in the FIG. 1).
SLIM employs a low cost large area organic substrate on which is defined a multiple level metal-polymer dielectric structure that provides power, ground, and controlled impedance interconnection functions together with a full range of integral passives and optoelectronics wave guide structures.
As previously noted, passives are non-active electrical elements which can be characterized in resistive, inductive and capacitive components. FIGS. 2(a)-(c) schematically shows the definition of discrete (a), integrated (b), and integral passives (c).
Among various functional passives, the ratio of decoupling capacitors can be more than 60%. On the other hand, increasing IC speed requires decoupling capacitors with higher capacitance and shorter distance from their serving components to increase switch performance. Thus integral decoupling capacitors can be a better approach in comparison to surface mounted discrete capacitors. One of the biggest constraints of organic based technology, however, is the low processing temperature required for subsequent layers. Many materials which have been reported to have high dielectric constants require processing under such high temperatures (e.g., sintering) that they are unsuitable for SLIM technology. Polymer-ceramic composites can be used in forming capacitors on organic substrates because they combine the low processing temperature of polymers and the high dielectric constant of ceramics. Chahal et al. obtained an epoxy-ceramic composite with 68% ceramic volume loading that has the dielectric constant of 74. Importantly, Chahal did not use a metal acetylacetonate as a curing catalyst for the epoxy. Normal commercial epoxy systems have a low dielectric constant of around 3-4, which hinders efforts to increase the effective dielectric constant of the composite.
Curing of epoxies is usually accomplished using a hardener and a catalyst. It is known in the art that acetylacetonates of various metals can be used for catalyzing the reaction between a polyepoxide and a polyanhydride or to catalyze the curing of an epoxide. Patents disclosing using a metal acetylacetonate to catalyze the cure of an epoxy material or a polyepoxy material mixed with a polycarboxylic acid or polyanhydride include U.S. Pat. Nos. 2,801,228, 3,424,699, 3,624,032, 3,812,214, 4,131,715, and 4,224,541.
Polymer-ceramic composites having a dielectric constant high than normal epoxy systems are described in U.S. Pat. No. 5,739,193. The disclosed composites contain thermoplastic polymers, two ceramic fillers one of which is mica, and have a dielectric constant of at least 4.0 at 1.0 GHz. High dielectric polymer-ceramic composites are generally made by combining a polymeric matrix and a ceramic filler having a dielectric constant higher than the polymeric matrix. This approach is difficult because the composites need high levels of the ceramic filler in order to achieve the desired dielectric constant while retaining rheological properties that make the composites suitable for extrusion or molding.
Materials based on polymers combined with ceramics also having dielectric constants higher than normal commercial epoxy systems are known in the art. For example, U.S. Pat. No. 4,335,180 discloses a polymer-ceramic composite comprising an anionic dispersion of poly(tetrafluoroethylene) with the particulate filler comprising titania and further having microfibrous material comprising glass microfibers. The dielectric constants were measured to be 10-11. U.S. Pat. No. 5,358,775 discloses a polmer ceramic composite having a fluoropolymer (preferably poly(tetrafluoroethylene)) filled with a particulate ceramic material (powder) which exhibits low loss, high dielectric constant (xe2x89xa730) and an acceptable low thermal coefficient of dielectric constant.
A high dielectric composite in which the matrix polymer is an epoxy resin based on bisphenol F epoxy and an organic amino curing agent and in which the filler is barium titanate at a 34 volume % level has been described (S. Asai, et al., IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. 16, No. 5, Aug., 1993, pp. 499-504). This composite is reportedly easy to process before the epoxy resins set because of the low viscosity of the epoxy prepolymer, and has dielectric constants up to about 20 and loss tangents of about 0.0165-0.0173 were observed with this composite.
An increase in capacitance can be attributed to the polarization introduced by the insulator material. There are several molecular mechanisms associated with this polarization: (a) electronic polarization; (b) atomic polarization; (c) dipole or orientation polarization; and (d) space-charge polarization. Electronic polarization is the result of dipole moments induced by the electric field. Atomic polarization results from an unsymmetrical sharing of electrons when two different atoms combine to form a molecule. Some molecules have permanent dipoles that can react with external electric field and form dipole or orientation polarization. Space-charge polarization is different from the previous three types of polarization in that it is not due to charges that are locally bound. Instead, it is produced by charge carriers, such as free ions, that can migrate for some distance throughout the dielectric. The dielectric constant or relative permittivity (∈r) represents the extent of polarization provided by the dielectric material. Most often the dielectric constant is measured as the ratio of the capacitance of a parallel-plate capacitor with the insulator material compared to the capacitance of the same capacitor with air.
The relationship between the capacitance C and the dielectric constant ∈r is given by the following equation:                     C        =                                            ϵ              0                        ⁢                          ϵ              r                        ⁢            A                    t                                    Equation  (1)            
where, ∈0 is dielectric constant of the free space (8.854xc3x9710xe2x88x9212 F/m), A is the area of the electrical conductor, t is the thickness of the insulator layer, and ∈r is the dielectric constant of the insulator layer. According to this relationship, the dielectric constant of insulator material should be as high as 114 in order to achieve capacitance of 20 nF/cm2, assuming a thickness of 5 xcexcm of insulate layer. Thus, developing an organic substrate compatible high dielectric constant material is a major challenge of integral capacitor technology.
Polymer-ceramic composites can be used in forming capacitors on organic substrates because they combine the processibility of polymers and the higher dielectric constant of ceramics. Polymers filled with ceramics have been studied for use as dielectric materials in thick film capacitors. Ceramic particle size has been proved to influence the effective dielectric constant of composite dramatically. The average radius of ceramic particle usually is less than 1 xcexcm, so that the composite is called a nano-composite.
Precisely predicting the effective dielectric constant of polymer-ceramic nano-composites is very important for the design of composite materials. Many theoretical models have been proposed in the literature for simulating the electrical and mechanical properties of such composites. In most cases, composite dielectrics are chaotic or statistical mixtures (randomly dispersed systems) of several components. The true value of permittivity of a statistic composite should lie between the values determined by Equation (2) as n=1 and n=xe2x88x921. In Equation (2), ∈ is the effective dielectric constant of the composite, vi is the volume of occupied by material i, and ∈i is the dielectric constant of material i.
The dielectric behavior of chaotic systems has been studied in great detail by many authors. Many generalized equations have been derived based on experimental results and theoretical derivation. The most commonly used equation is the Lichtenecker logarithmic law of mixing and is written for a two component system as shown in Equation (3). Equation (4) is a modified form of Lichtenecker equation, where k is a fitting constant subject to composite material. It is reported that k has value around 0.3 for most well-dispersed polymer-ceramic composites.                               ϵ          n                =                              ∑                          i              =              1                        m                    ⁢                      (                                          v                i                            ⁢                              ϵ                i                n                                      )                                              Equation  (2)                                          log          ⁢                      xe2x80x83                    ⁢          ϵ                =                                            v              1                        ⁢            log            ⁢                          xe2x80x83                        ⁢                          ϵ              1                                +                                    v              2                        ⁢            log            ⁢                          xe2x80x83                        ⁢                          ϵ              2                                                          Equation  (3)                                          log          ⁢                      xe2x80x83                    ⁢          ϵ                =                              log            ⁢                          xe2x80x83                        ⁢                          ϵ              1                                +                                                    v                2                            ⁡                              (                                  1                  -                  k                                )                                      ⁢            log            ⁢                          xe2x80x83                        ⁢                          (                                                ϵ                  2                                                  ϵ                  1                                            )                                                          Equation  (4)            
The models mentioned above are generally either too simple to properly describe the polymer-ceramic nano-composite property or are merely empirical models. For example, Lichtenecker logarithmic law needs to obtain fitting factor k every time materials are changed, because k is sensitive to both polymer and ceramic materials.
It has been discovered that base polymers containing a metal acetylacetonate (acacs) can be combined with additional substances, preferably ceramic fillers, to form two phase composites having high dielectric constants. Exemplary base polymers include but are not limited to epoxies. In particular, it has been discovered that 5 weight percent Co(III) acac can increase the dielectric constant of DER661 epoxy by about 60%. Weight percent equals the weight of solute divided by the weight of solution multiplied by 100. Composites having about 30 to about 90% volume ceramic loading and a high dielectric base polymer, preferably epoxy, have been discovered to have a dielectric constants greater than about 60. In an exemplary aspect of the present invention, composites having about 30 to about 90% volume ceramic loading and a high dielectric base polymer, preferably epoxy, have been discovered to have a dielectric constants greater than about 74 to about 150. In one aspect of the present invention, high dielectric constant base polymers, preferably epoxy, as a matrix, combined with ceramic fillers obtained a dielectric constant of about 98 to about 150 at a frequency of at least 10 kHz. In another aspect, the dielectric constant is greater than 100 at a frequency of at least 10 kHz. The dielectric constants of the compositions of the present invention exceed the dielectric constants of known base polymers and composites by at about 30% to about 80%.
In yet another aspect of the present invention, a prototype embedded capacitor with capacitance density of at least 25 nF/cm2, preferably at least 35 nF/cm2, most preferably about 50 nF/cm2 using the novel high dielectric constant composites described herein is disclosed.
In other aspects of the present invention, methods for forming two phase composites having high dielectric constants include: dissolving a metal acetylacetonate into a polymer resin; adding a hardener to form a base polymer containing a metal acetylacetonate; and combining a ceramic filler with the base polymer. The volume loading of filler, preferably ceramic filler, can be about 30% to about 90%, preferably about 60 to about 85%. In a preferred aspect, the average size of the filler particles are less than 1 xcexcm.
It is an object of the present invention to provide novel polymer-ceramic composite systems having high dielectric constants suitable for use in embedded capacitor applications.
It is still another object of the present invention to provide methods for increasing the dielectric constant of two phase composites.
It is yet another object of the present invention to provide improved embedded capacitors and methods for making said improved embedded capacitors.
Other objects, aspects, and advantages of the present invention will be apparent to those skilled in the art from a reading of the following detailed disclosure of the invention.